Field of the Invention
The present invention relates in general to a sequential selection circuit and more particularly to a protective circuit suitable for scanning the cathodes and comprising a selection circuit of a matrix type plasma display.
Description of the Prior Art
Plasma display panels of an X-Y matrix type are known as a means for displaying characters or images. An X-electrode group (data electrodes) comprises anodes to which high and low voltages are applied according to the display data. A Y-electrode group (scanning electrodes) comprise cathodes which are scanned in a line sequential manner with a negative voltage pulse being applied.
A cathode scanning circuit comprises a shift register having the same number of shift stages as the number of cathodes and a group of drive transistors which are sequentially turned on in response to outputs from respective stages of the shift register so as to apply voltages to the cathodes. If it be assumed that a pulse of a scanning period of the cathode is an H pulse which is the horizontal scanning pulse and a pulse of one frame period (one frame) is a V pulse which is the vertical scanning pulse, the shift register receives a V pulse of high level and sequentially transfers it to succeeding stages using the H pulses as clock signals. Since the V pulse returns to a low level immediately after being received by the shift register, single data is always shifted in the shift register through the stages one by one so that the cathodes are sequentially activated.
However, if the V pulse of high level becomes an abnormally long pulse width or when the V pulse is fixed at a high level due to short circuiting of a circuit or element, a plurality of stages in the shift register will simultaneously be set to "1" and a plurality of cathodes will be selected at the same time. In this case, current through the discharge cells will be considerably increased and power supply units or drive transistors of the cathode drive circuit can be damaged.